In the presence of several different ions, the equilibrium of the cell
depends on the relative permeability of the ions. For this, we use
the Goldman-Hodgkin-Katz equation:

(2)

Permeability of an ion is dependent on a number of factors such as the
size of the ion, its mobility, etc.
During rest in the squid giant axon, the permeabilities have the ratio
PK:PNa:PCl = 1:0.03:0.1 so that

Since PK dominates, this is close to EK.
During an action potential the ratio is PK:PNa:PCl =1:15:.1
so that

Later on we will approximate the GHK equations by a linearized
version:

where the conductances g are proportional to the permeabilities.

HOMEWORK

1.

Suppose the external potassium in a mammalian cell is increased by a
factor of 10. What is the new value of EK?

2.

At a cell contains 80 mM sodium inside and has only
100 mM sodium outside. What is the equilibrium potential for sodium?

3.

Using the same permeabilities for the mammalian cell as were
used for the squid axon, compute Vrest,Vm using the table
below.